The present invention relates to an improvement in organ pacer sensing systems and, more specifically, to a system which allows short-duration pulses to be monitored over a communication line having only a low frequency transmission capability.
Various types of electrical organ stimulators have been developed and adapted to provide the stimuli needed to cause a defective or damaged organ to function properly. The most widely used of this type of apparatus is the battery powered electronic cardiac stimulator or "heart pacer" which is conventionally implanted beneath the skin of a user. Such implanted pacers are ordinarily self-contained and powered by a battery within the pacer casing. The pacer utilizes energy from a battery to generate electrical pulses which are applied via conductive leads to predetermined portions of an organ, such as a heart. The pulses generated by the pacer comprise artificial stimulation signals which supplant the periodic electrical stimulation signals which are naturally generated within the body for stimulating the organ. In general, a heart is electrically stimulated to beat once for each pulse which is generated by the pacer.
A number of different types of heart pacers have been developed and, while they are alike in that they supply electrical stimulation in a predetermined fashion, the timing and nature of the pulses may vary from one pacer to another. More important to the present description, the response of pacer operation to a decline in battery voltage, caused by faulty or aging battery, may differ substantially from one type of pacer to another.
The types of batteries which are best suited for energizing pacers conventionally exhibit a constant output voltage throughout most of their operative lives, then near the end of their lives the voltage declines in a relatively short period. With most types of pacer circuits, the pulse rate of the pacer decreases toward the end of battery life. Further, in one type of pacer the width of the output pulses increases as a result of decrease in battery voltage. With most types of pacers, however, the width or duration of the outputted pulses decreases as a function of battery voltage. Accordingly, the width of a pacer pulse is a valuable indication of battery voltage, and may also be used as a confirmation of the status of the battery.
In some modes of improper activity or failure of pacer circuitry the pulse rate may change, despite the fact that the status of the battery remains the same. At the same time, however, the width of the pulse will normally remain constant. Accordingly, a measure of the pulse width can serve to distinguish between a change in pulse rate due to circuit failure, and one due to a decline in battery charge.
Some newer types of pacers maintain a substantially constant pulse rate despite a decline in battery voltage; however, the change in pulse width is substantial and accordingly pulse width monitoring is the only feasible way of monitoring battery status. In such a pacer, the system is adapted to output a substantially constant burst of energy with each pulse so that as amplitude declines with battery aging, pulse width is caused to increase.
Apparatus are now known for detecting and transducing pacer signals, and coupling them to telephonic communication links so that they may be received and analyzed by equipment at a remote location. See for instance the abstract entitled "Transtelephone Pacemaker Clinic" by S. Furman, B. Parker and D. Escher, at page 94 of the American Journal of Cardiology, Vol. 25. Such systems are further described in U.S. Pat. Nos. 3,872,252 -- Malchman et al. and 3,826,246 -- Raddi et al. With such systems a pacer wearer can allow the battery in his implanted pacer to be checked regularly by a cardiologist or clinician at a distant location without the need for actual visits to the physician or clinician.
In the systems of interest pacer output pulses and/or cardiac signals are sensed by appropriate transducers and converted to acoustical signals which are transmittable by a telephone communication link. As is well known, commercial telephone links are responsive over only a narrow frequency band, extending from approximately 300 to 3000 Hz. While this relatively narrow band is adequate for the transmission of voice grade signals, it places severe constraints upon the transmission of more complex information. In the instance of interest, the pacer pulses which are desired to be monitored exhibit a duration or width of from approximately one-half to two milliseconds, and accordingly cannot be resolved over telephonic links with sufficient accuracy to allow changes in pulse duration to be detected.
Accordingly, it will be seen that it would be highly desirable to provide a system for producing telephonically transmittable signals which accurately represent the pulse width of electric pacer pulses.
It is accordingly an object of the present invention to provide means for converting a detected pacer pulse into an acoustically transmissible signal.
Another object of the invention is to provide a circuit responsive to a high frequency pacer pulse to produce a lower-frequency representation thereof.
Still another object is to provide an improved monitoring means for producing a representation of the duration of detected pacer pulses which can be transmitted over conventional telephone equipment.